Internet DRAFT - draft-ietf-trill-clear-correct
draft-ietf-trill-clear-correct
TRILL Working Group Donald Eastlake
INTERNET-DRAFT Mingui Zhang
Intended status: Proposed Standard Huawei
Updates: 6325, 6327, 6439 Anoop Ghanwani
Dell
Vishwas Manral
Hewlett-Packard
Ayan Banerjee
Cumulus Networks
Expires: January 29, 2013 July 30, 2012
TRILL: Clarifications, Corrections, and Updates
<draft-ietf-trill-clear-correct-06.txt>
Abstract
The IETF TRILL (TRansparent Interconnection of Lots of Links)
protocol provides least cost pair-wise data forwarding without
configuration in multi-hop networks with arbitrary topology and link
technology, safe forwarding even during periods of temporary loops,
and support for multipathing of both unicast and multicast traffic.
TRILL accomplishes this by using IS-IS (Intermediate System to
Intermediate System) link state routing and by encapsulating traffic
using a header that includes a hop count. Since the TRILL base
protocol was approved in March 2010, active development of TRILL has
revealed errata in the original RFC 6325 and some cases that could
use clarifications or updates.
RFC 6327 and RFC 6439 provide clarifications and updates with respect
to Adjacency and Appointed Forwarders. This document provides other
known clarifications, corrections, and updates to RFC 6325, RFC 6327,
and RFC 6439.
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. Distribution of this document is
unlimited. Comments should be sent to the TRILL working group
mailing list <rbridge@postel.org>.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
D. Eastlake, et al [Page 1]
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft
Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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Table of Contents
1. Introduction............................................4
1.1 Precedence.............................................4
1.2 Changes That Are Not Backwards Compatible..............4
1.3 Terminology and Acronyms...............................5
2. Overloaded and/or Unreachable RBridges..................6
2.1 Reachability...........................................6
2.2 Distribution Trees.....................................7
2.3 Overloaded Receipt of TRILL Data Frames................7
2.3.1 Known Unicast Receipt................................7
2.3.2 Multi-Destination Receipt............................8
2.4 Overloaded Origination of TRILL Data Frames............8
2.4.1 Known Unicast Origination............................8
2.4.2 Multi-Destination Origination........................8
2.4.2.1 An Example Network.................................8
2.4.2.2 Indicating OOMF Support............................9
2.4.2.3 Using OOMF Service................................10
3. Distribution Trees.....................................11
3.1 Number of Distribution Trees..........................11
3.2 Clarification of Distribution Tree Updates............11
3.3 IP Address Based Multicast Pruning....................11
3.4 Numbering of Distribution Trees.......................12
4. Nickname Selection.....................................13
5. MTU (Maximum Transmission Unit)........................15
5.1 MTU Related Errata in RFC 6325........................15
5.1.1 MTU PDU Addressing..................................15
5.1.2 MTU PDU Processing..................................16
5.1.3 MTU Testing.........................................16
5.2 Ethernet MTU Values...................................16
6. Port Modes.............................................18
7. The CFI / DEI Bit......................................19
8. Graceful Restart.......................................20
9. Updates to RFC 6327....................................21
10. Updates on Appointed Forwarders and Inhibition........22
10.1 Optional TRILL Hello Reduction.......................22
10.2 Overload and Appointed Forwarders....................24
11. IANA Considerations...................................25
12. Security Considerations...............................26
Acknowledgements..........................................26
Change History............................................26
Normative References......................................28
Informative References....................................29
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1. Introduction
The IETF TRILL (Transparent Interconnection of Lots of Links)
protocol [RFC6325] provides optimal pair-wise data frame forwarding
without configuration in multi-hop networks with arbitrary topology
and link technology, safe forwarding even during periods of temporary
loops, and support for multipathing of both unicast and multicast
traffic. TRILL accomplishes this by using IS-IS (Intermediate System
to Intermediate System) [IS-IS] [RFC1195] [RFC6326bis] link state
routing and encapsulating traffic using a header that includes a hop
count. The design supports VLANs (Virtual Local Area Networks) and
optimization of the distribution of multi-destination frames based on
VLANs and IP derived multicast groups.
In the more than two years since the TRILL base protocol [RFC6325]
was approved, the active development of TRILL has revealed five
errors in the original specification document [RFC6325] and cases
that could use clarifications or updates.
[RFC6327] and [RFC6439] provide clarifications with respect to
Adjacency and Appointed Forwarders. This document provides other
known clarifications, corrections, and updates to [RFC6325],
[RFC6327], and [RFC6439].
1.1 Precedence
In case of conflict between this document and any of [RFC6325],
[RFC6327], or [RFC6439], this document takes precedence. In addition,
Section 1.2 (Normative Content and Precedence) of [RFC6325] is
updated to provide a more complete precedence ordering of the
sections of [RFC6325] as following, where sections to the left take
precedence over sections to their right:
4 > 3 > 7 > 5 > 2 > 6 > 1
1.2 Changes That Are Not Backwards Compatible
The change made by Section 3.4 below is not backward compatible with
[RFC6325] but has nevertheless been adopted to reduce distribution
tree changes resulting from topology changes.
The several other changes herein that are fixes to previously posted
[RFC6325] Errata [Err3002] [Err3003] [Err3004] [Err3052] [Err3053]
may not be backward compatible with previous implementations that
conformed to errors in the specification.
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1.3 Terminology and Acronyms
This document uses the acronyms defined in [RFC6325] and the
following additional acronyms:
CFI - Canonical Format Indicator [802]
DEI - Drop Eligibility Indicator [802.1Q-2011]
EISS - Enhanced Internal Sublayer Service
OOMF - Overload Originated Multi-destination Frame
TRILL Switch - An alternative name for an RBridge
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
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2. Overloaded and/or Unreachable RBridges
RBridges may be in overload as indicated by the [IS-IS] overload flag
in their LSPs (Link State PDUs (Protocol Data Units)). This means
that either (1) they are incapable of holding the entire link state
database and thus do not have a view of the entire topology or (2)
they have been configured to have the overload bit set. Although
networks should be engineered to avoid actual link state overload, it
might occur under various circumstances. For example, if a large
campus included one or more low-end TRILL Switches.
It is a common operational practice to set the overload bit in an
[IS-IS] router (such as an RBridge) when performing maintenance on
that router that might affect its ability to correctly forward
frames; this will usually leave the router reachable for maintenance
traffic but transit traffic will not be routed through it. (Also, in
some cases, TRILL provides for setting the overload bit in the
pseudonode of a link to stop TRILL Data traffic on an access link
(see Section 4.9.1 of [RFC6325]).)
[IS-IS] and TRILL make a reasonable effort to do what they can even
if some RBridges/routers are in overload. They can do reasonably well
if a few scattered nodes are in overload. However, actual least cost
paths are no longer assured if any RBridges are in overload.
For the effect of overload on the appointment of forwarders, see
Section 10.2.
In this Section 2, the term "neighbor" refers only to actual RBridges
and ignores pseudonodes.
2.1 Reachability
Frames are not least cost routed through an overloaded TRILL Switch,
although they may originate or terminate at an overloaded TRILL
Switch. In addition, frames will not be least cost routed over links
with cost 2**24 - 1 [RFC5305]; such links are reserved for traffic
engineered frames, the handling of which is beyond the scope of this
document.
As a result, a portion of the campus may be unreachable for least
cost routed TRILL Data because all paths to it would be through a
link with cost 2**24 - 1 or through an overloaded RBridge. For
example, an RBridge RB1 is not reachable by TRILL Data if all of its
neighbors are connected to RB1 by links with cost 2**24 - 1. Such
RBridges are called "data unreachable".
The link state database at an RBridge RB1 can also contain
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information on TRILL Switches that are unreachable by IS-IS link
state flooding due to link or RBridge failures. When such failures
partition the campus, the TRILL Switches adjacent to the failure and
on the same side of the failure as RB1 will update their LSPs to show
the lack of connectivity and RB1 will receive those updates. As a
result, RB1 will be aware of the partition. Nodes on the far side of
the partition are both "IS-IS unreachable" and data unreachable.
However, LSPs held by RB1 for TRILL Switches on the far side of the
failure will not be updated and may stay around until they time out,
which could be tens of minutes or longer. (The default in [IS-IS] is
twenty minutes.)
2.2 Distribution Trees
An RBridge in overload cannot be trusted to correctly calculate
distribution trees or correctly perform the RPFC (Reverse Path
Forwarding Check). Therefore, it cannot be trusted to forward multi-
destination TRILL Data frames. It can only appear as a leaf node in a
TRILL multi-destination distribution tree. Furthermore, if all the
immediate neighbors of an RBridge are overloaded, then it is omitted
from all trees in the campus and is unreachable by multi-destination
frames.
When an RBridge determines what nicknames to use as the roots of the
distribution trees it calculates, it MUST ignore all nicknames held
by TRILL Switches that are in overload or are data unreachable. When
calculating RPFCs for multi-destination frames, an RBridge RB1 MAY,
to avoid calculating unnecessary RPF check state, ignore any trees
that cannot reach to RB1 even if other RBridges list those trees as
trees those other TRILL Switches might use. (But see Section 3.)
2.3 Overloaded Receipt of TRILL Data Frames
The receipt of TRILL Data frames by overloaded RBridge RB2 is
discussed in the subsections below. In all cases, the normal Hop
Count decrement is performed and the TRILL Data frame is discarded if
the result is less than one or if the egress nickname is illegal.
2.3.1 Known Unicast Receipt
RB2 will not usually receive unicast TRILL Data frames unless it is
the egress, in which case it decapsulates and delivers the frames
normally. If RB2 receives a unicast TRILL Data frame for which it is
not the egress, perhaps because a neighbor does not yet know it is in
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overload, RB2 MUST NOT discard the frame because the egress is an
unknown nickname as it might not know about all nicknames due to its
overloaded condition. If any neighbor, other than the neighbor from
which it received the frame, is not overloaded it MUST attempt to
forward the frame to one of those neighbors. If there is no such
neighbor, the frame is discarded.
2.3.2 Multi-Destination Receipt
If RB2 in overload receives a multi-destination TRILL Data frame, RB2
MUST NOT apply an RPFC since, due to overload, it might not do so
correctly. RB2 decapsulates and delivers the frame locally where it
is Appointed Forwarder for the frame's VLAN, subject to any multicast
pruning. But since, as stated above, RB2 can only be the leaf of a
distribution tree, it MUST NOT forward a multi-destination TRILL Data
frame (except as an egressed native frame where RB2 is Appointed
Forwarder).
2.4 Overloaded Origination of TRILL Data Frames
Overloaded origination of unicast frames with known egress and of
multi-destination frames are discussed in the subsections below.
2.4.1 Known Unicast Origination
When an overloaded RBridge RB2 ingresses or creates a known
destination unicast TRILL Data frame, it delivers it locally if the
destination MAC is local. Otherwise RB2 unicasts it to any neighbor
TRILL Switch that is not overloaded. It MAY use what routing
information it has to help select the neighbor.
2.4.2 Multi-Destination Origination
Overloaded RBridge RB2 ingressing or creating a multi-destination
TRILL Data frame is more complex than for a known unicast frame.
2.4.2.1 An Example Network
For example, consider the network below in which, for simplicity, end
stations and any bridges are not shown. There is one distribution
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tree of which RB4 is the root and which is represented by double
lines. Only RBridge RB2 is overloaded.
+-----+ +-----+ +-----+ +-----+
| RB7 +====+ RB5 +=====+ RB3 +=====+ RB1 |
+-----+ +--+--+ +-++--+ +--+--|
| || |
+---+---+ || |
+------+RB2(ov)|======++ |
| +-------+ || |
| || |
+--+--+ +-----+ ++==++=++ +--+--+
| RB8 +=====+ RB6 +==++ RB4 ++=====+ RB9 |
+-----+ +-----+ ++=====++ +-----+
Since RB2 is overloaded it does not know what the distribution tree
or trees are for the network. Thus there is no way it can provide
normal TRILL Data encapsulation for multi-destination native frames.
So RB2 tunnels the frame to a neighbor that is not overloaded if it
has such a neighbor that has signaled it is willing to offer this
service. RBridges indicate this in their Hellos as described below.
This service is called OOMF (Overloaded Origination of Multi-
destination Frame) service.
- The multi-destination frame MUST NOT be locally distributed in
native form at RB2 before tunneling to a neighbor because this
would cause the frame to be delivered twice. For example, if RB2
locally distributed a multicast native frame and then tunneled it
to RB5, RB2 would get a copy of the frame when RB3 transmitted it
as a TRILL Data frame on the multi-access RB2-RB3-RB4 link. Since
RB2 would, in general, not be able to tell that this was a frame
it had tunneled for distribution, RB2 would decapsulate it and
locally distribute it a second time.
- On the other hand, if there is no neighbor of RB2 offering RB2 the
OOMF service, RB2 cannot tunnel the frame to a neighbor. In this
case RB2 MUST locally distribute the frame where it is Appointed
Forwarder for the frame's VLAN and optionally subject to multicast
pruning.
2.4.2.2 Indicating OOMF Support
A RBridge RB3 indicates its willingness to offer the OOMF service to
RB2 in the TRILL Neighbor TLV in RB3's TRILL Hellos by setting a bit
associated with the SNPA (SubNetwork Point of Attachment, also known
as MAC address) of RB2 on the link. (See Section 11.) Overloaded
RBridge RB2 can only distribute multi-destination TRILL Data frames
to the campus if a neighbor of RB2 not in overload offers RB2 the
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OOMF service. If RB2 does not have OOMF service available to it, RB2
can still receive multi-destination frames from non-overloaded
neighbors and, if RB2 should originate or ingress such a frame, it
distributes it locally in native form.
2.4.2.3 Using OOMF Service
If RB2 sees this OOMF (Overloaded Origination of Multi-destination
Frame) service advertised for it by any of its neighbors on any link
to which RB2 connects, it selects one such neighbor by a means beyond
the scope of this document. Assuming RB2 selects RB3 to handle multi-
destination frames it originates. RB2 MUST advertise in its LSP that
it might use any of the distribution trees that RB3 advertises so
that the RPFC will work in the rest of the campus. Thus,
notwithstanding its overloaded state, RB2 MUST retain this
information from RB3 LSPs, which it will receive as it is directly
connected to RB3.
RB2 then encapsulates such frames as TRILL Data frames to RB3 as
follows: M bit = 0, Hop Count = 2, ingress nickname = a nickname held
by RB2, and, since RB2 cannot tell what distribution tree RB3 will
use, egress nickname = a special nickname indicating an OOMF frame
(see Section 11). RB2 then unicasts this TRILL Data frame to RB3.
(Implementation of Item 4 in Section 4 below provides reasonable
assurance that, notwithstanding its overloaded state, the ingress
nickname used by RB2 will be unique within at least the portion of
the campus that is IS-IS reachable from RB2.)
On receipt of such a frame, RB3 does the following:
- change the egress nickname field to designate a distribution tree
that RB3 normally uses,
- set the M bit to one,
- change the Hop Count to the value it would normally use if it were
the ingress, and
- forward the frame on that tree.
RB3 MAY rate limit the number of frames for which it is providing
this service by discarding some such frames from RB2. The provision
of even limited bandwidth for OOMFs by RB3, perhaps via the slow
path, may be important to the bootstrapping of services at RB2 or at
end stations connected to RB2, such as supporting DHCP and ARP/ND.
(Everyone sometimes needs a little OOMF (pronounced oomph) to get off
the ground.)
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3. Distribution Trees
A correction, a clarification, and two updates related to
distribution trees appear in the subsections below. See also Section
2.2.
3.1 Number of Distribution Trees
In [RFC6325], Section 4.5.2, page 56, Point 2, 4th paragraph, the
parenthetical "(up to the maximum of {j,k})" is incorrect [Err3052].
It should read "(up to k if j is zero or the minimum of (j, k) if j
is non-zero)".
3.2 Clarification of Distribution Tree Updates
When a link state database change causes a change in the distribution
tree(s), there are several possibilities. If a tree root remains a
tree root but the tree changes, then local forwarding and RPFC
entries for that tree should be updated as soon as practical.
Similarly, if a new nickname becomes a tree root, forwarding and RPFC
entries for the new tree should be installed as soon as practical.
However, if a nickname ceases to be a tree root and there is
sufficient room in local tables, the forwarding and RPFC entries for
the former tree MAY be retained so that any multi-destination TRILL
Data frames already in flight on that tree have a higher probability
of being delivered.
3.3 IP Address Based Multicast Pruning
The TRILL base protocol specification [RFC6325] provides for and
recommends the pruning of multi-destination frame distribution trees
based on the location of IP multicast routers and listeners; however,
multicast listening is identified by derived MAC addresses as
communicated in the Group MAC Address sub-TLV [RFC6326bis].
TRILL Switches MAY communicate multicast listeners and prune
distribution trees based on the actual IPv4 or IPv6 multicast
addresses involved. Additional Group Address sub-TLVs are provided in
[RFC6326bis] to carry this information. A TRILL Switch that is only
capable of pruning based on derived MAC address SHOULD calculate and
use such derived MAC addresses from multicast listener IPv4/IPv6
address information it receives.
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3.4 Numbering of Distribution Trees
Section 4.5.1 of [RFC6325] specifies that, when building distribution
tree number j, node (RBridge) N that has multiple possible parents in
the tree is attached to possible parent number j mod p. Trees are
numbered starting with 1 but possible parents are numbered starting
with 0. As a result, if there are two trees and two possible parents,
in tree 1 parent 1 will be selected and in tree 2 parent 0 will be
selected.
This is changed so that the selected parent MUST be (j-1) mod p. As a
result, in the case above, tree 1 will select parent 0 and tree 2
will select parent 1. This change is not backwards compatible with
[RFC6325]. If all RBridges in a campus do not determine distribution
trees in the same way then, for most topologies, the reverse path
forwarding checking will drop many multi-destination frames before
they have been properly delivered.
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4. Nickname Selection
Nickname selection is covered by Section 3.7.3 of [RFC6325]. However,
the following should be noted:
1. The second sentence in the second bullet item in Section 3.7.3 of
[RFC6325] on page 25 is erroneous [Err3002] and is corrected as
follows:
1.a The occurrence of "IS-IS ID (LAN ID)" is replaced with
"priority".
1.b The occurrence of "IS-IS System ID" is replaced with "seven
byte IS-IS ID (LAN ID)".
The resulting corrected [RFC6325] sentence reads as follows: "If
RB1 chooses nickname x, and RB1 discovers, through receipt of an
LSP for RB2 at any later time, that RB2 has also chosen x, then
the RBridge or pseudonode with the numerically higher priority
keeps the nickname, or if there is a tie in priority, the RBridge
with the numerically higher seven byte IS-IS ID (LAN ID) keeps the
nickname, and the other RBridge MUST select a new nickname."
2. In examining the link state database for nickname conflicts,
nicknames held by IS-IS unreachable TRILL Switches MUST be ignored
but nicknames held by IS-IS reachable TRILL Switches MUST NOT be
ignored even if they are data unreachable.
3. An RBridge may need to select a new nickname, either initially
because it has none or because of a conflict. When doing so, the
RBridge MUST consider as available all nicknames that do not
appear in its link state database or that appear to be held by IS-
IS unreachable TRILL Switches; however, it SHOULD give preference
to selecting new nicknames that do not appear to be held by any
TRILL Switch in the campus, reachable or unreachable, so as to
minimize conflicts if IS-IS unreachable TRILL Switches later
become reachable.
4. An RBridge, even after it has acquired a nickname for which there
appears to be no conflicting claimant, MUST continue to monitor
for conflicts with the nickname or nicknames it holds. It does so
by checking in LSPs it receives that should update its link state
database for any of its nicknames held with higher priority by
another TRILL Switch that is IS-IS reachable. If it finds such a
conflict, it MUST select a new nickname, even when in overloaded
state. (It is possible to receive an LSP that should update the
link state database but does not due to overload.)
5. In the very unlikely case that an RBridge is unable to obtain a
nickname because all valid RBridge nicknames (0x0001 through
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0xFFBF inclusive) are in use with higher priority by IS-IS
reachable TRILL Switches, it will be unable to act as an ingress,
egress, or tree root but will still be able to function as a
transit TRILL Switch. Although it cannot be a tree root, such an
RBridge is included in distribution trees computed for the campus
unless all its neighbors are overloaded. It would not be possible
to send a unicast RBridge Channel message specifically to such a
TRILL Switch [Channel]; however, it will receive unicast Channel
messages sent by a neighbor to the Any-RBridge egress nickname and
will receive appropriate multi-destination Channel messages.
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5. MTU (Maximum Transmission Unit)
MTU values in TRILL key off the originatingL1LSPBufferSize value
communicated in the IS-IS originatingLSPBufferSize TLV [IS-IS]. The
campus-wide value Sz, as described in [RFC6325] Section 4.3.1, is the
minimum value of originatingL1LSPBufferSize for the RBridges in a
campus, but not less than 1470. The MTU testing mechanism and
limiting LSPs to Sz assures that the LSPs can be flooded by IS-IS and
thus that IS-IS can operate properly.
If nothing is known about the MTU of the links or the
originatingL1LSPBufferSize of other RBridges in a campus, the
originatingL1LSPBufferSize for an RBridge should default to the
minimum of the LSP size that its TRILL IS-IS software can handle and
the minimum MTU of the ports that it might use to receive or transmit
LSPs. If an RBridge does have knowledge of link MTUs or other RBridge
originatingL1LSPBufferSize then, to avoid the necessity to regenerate
the local LSPs using a different maximum size, the RBridge's
originatingL1LSPBufferSize SHOULD be configured to the minimum of (1)
the smallest value that other RBridges are or will be announcing as
their originatingL1LSPBufferSize and (2) a value small enough that
the campus will not partition due to a significant number of links
with limited MTU. However, as provided in [RFC6325], in no case can
originatingL1LSPBufferSize be less than 1470. In a well configured
campus, to minimize any LSP regeneration due to re-sizing, it is
desirable for all RBridges to be configured with the same
originatingL1LSPBufferSize.
Section 5.1 below corrects errata in [RFC6325] and Section 5.2
clarifies the meaning of various MTU (Maximum Transmission Unit)
limits for TRILL Ethernet links.
5.1 MTU Related Errata in RFC 6325
Three MTU related errata in [RFC6325] are corrected in the
subsections below.
5.1.1 MTU PDU Addressing
Section 4.3.2 of [RFC6325] incorrectly states that multi-destination
MTU-probe and MTU-ack TRILL IS-IS PDUs are sent on Ethernet links
with the All-RBridges multicast address as the Outer.MacDA [Err3004].
As TRILL IS-IS PDUs, when multicast on an Ethernet link, they MUST be
sent to the All-IS-IS-RBridges multicast address.
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5.1.2 MTU PDU Processing
As discussed in [RFC6325] and, in more detail, in [RFC6327], MTU-
probe and MTU-ack PDUs MAY be unicast; however, Section 4.6 of
[RFC6325] erroneously does not allow for this possibility [Err3003].
It is corrected by replacing Item numbered "1" in Section 4.6.2 of
[RFC6325] with the following quoted text to which TRILL Switches MUST
conform:
"1. If the Ethertype is L2-IS-IS and the Outer.MacDA is either All-
IS-IS-RBridges or the unicast MAC address of the receiving
RBridge port, the frame is handled as described in Section
4.6.2.1"
The reference to "Section 4.6.2.1" in the above quoted text is to
that Section in [RFC6325].
5.1.3 MTU Testing
The last two sentences of Section 4.3.2 of [RFC6325] have errors
[Err3053]. They currently read:
If X is not greater than Sz, then RB1 sets the "failed minimum MTU
test" flag for RB2 in RB1's Hello. If size X succeeds, and X > Sz,
then RB1 advertises the largest tested X for each adjacency in the
TRILL Hellos RB1 sends on that link, and RB1 MAY advertise X as an
attribute of the link to RB2 in RB1's LSP.
They should read:
If X is not greater than or equal to Sz, then RB1 sets the "failed
minimum MTU test" flag for RB2 in RB1's Hello. If size X succeeds,
and X >= Sz, then RB1 advertises the largest tested X for each
adjacency in the TRILL Hellos RB1 sends on that link, and RB1 MAY
advertise X as an attribute of the link to RB2 in RB1's LSP.
5.2 Ethernet MTU Values
originatingL1LSPBufferSize is the maximum permitted size of LSPs
starting with the 0x83 Intradomain Routeing Protocol Discriminator
byte. In layer 3 IS-IS, originatingL1LSPBufferSize defaults to 1492
bytes. (This is because, in its previous life as DECnet Phase V, IS-
IS was encoded using the SNAP SAP [RFC5342] format which takes 8
bytes of overhead and 1492 + 8 = 1500, the classic Ethernet maximum.
When standardized by ISO/IEC [IS-IS] to use LLC encoding, this
default could have been increased by a few bytes but was not.)
D. Eastlake, et al [Page 16]
INTERNET-DRAFT TRILL: Clarifications and Corrections
In TRILL, originatingL1LSPBufferSize defaults to 1470 bytes. This
allows 27 bytes of headroom or safety margin to accommodate legacy
devices with the classic Ethernet maximum MTU despite headers such as
an Outer.VLAN. This safety margin is called "Margin" below.
Assuming the campus wide minimum link MTU is Sz, RBridges on Ethernet
links MUST limit most TRILL IS-IS PDUs so that PDUz (the length of
the PDU starting just after the L2-IS-IS Ethertype and ending just
before the Ethernet frame FCS) does not to exceed Sz. The PDU
exceptions are TRILL Hello PDUs, which MUST NOT exceed 1470 bytes,
and MTU-probe and MTU-ack PDUs that are padded, depending on the size
being tested (which may exceed Sz).
Sz does not limit TRILL Data frames. They are only limited by the MTU
of the devices and links that they actually pass through; however,
links that can accommodate IS-IS PDUs up to Sz would accommodate,
with a generous safety margin, TRILL Data frame payloads, starting
after the Inner.VLAN and ending just before the FCS, of Sz - 24
bytes. Most modern Ethernet equipment has ample headroom for frames
with extensive headers and is sometimes engineered to accommodate 9K
byte jumbo frames.
D. Eastlake, et al [Page 17]
INTERNET-DRAFT TRILL: Clarifications and Corrections
6. Port Modes
Section 4.9.1 of [RFC6325] specifies four mode bits for RBridge ports
but may not be completely clear on the effects of various
combinations of bits.
The table below explicitly indicates the effect of all possible
combinations of the TRILL port mode bits. "*" in one of the first
four columns indicates that the bit can be either zero or one. The
following columns indicate allowed frame types. The Disable bit
normally disables all frames but, as an implementation choice, some
or all low level Layer 2 control frames (as specified in [RFC6325]
Section 1.4) can still be sent or received.
+-+-+-+-+--------+-------+-----+-----+-----+
|D| | | | | | | | |
|i| |A| | | |TRILL| | |
|s| |c|T| | |Data | | |
|a| |c|r| | | | | |
|b|P|e|u| |native | LSP | | |
|l|2|s|n|Layer 2 |ingress| SNP |TRILL| P2P |
|e|P|s|k|Control |egress | MTU |Hello|Hello|
+-+-+-+-+--------+-------+-----+-----+-----+
|0|0|0|0| Yes | Yes | Yes | Yes | No |
+-+-+-+-+--------+-------+-----+-----+-----+
|0|0|0|1| Yes | No | Yes | Yes | No |
+-+-+-+-+--------+-------+-----+-----+-----+
|0|0|1|0| Yes | Yes | No | Yes | No |
+-+-+-+-+--------+-------+-----+-----+-----+
|0|0|1|1| Yes | No | No | Yes | No |
+-+-+-+-+--------+-------+-----+-----+-----+
|0|1|0|*| Yes | No | Yes | No | Yes |
+-+-+-+-+--------+-------+-----+-----+-----+
|0|1|1|*| Yes | No | No | No | Yes |
+-+-+-+-+--------+-------+-----+-----+-----+
|1|*|*|*|Optional| No | No | No | No |
+-+-+-+-+--------+-------+-----+-----+-----+
(The formal name of the "access bit" is the "TRILL traffic disable
bit" and the formal name of the "trunk bit" is the "end-station
service disable bit" [RFC6325].)
D. Eastlake, et al [Page 18]
INTERNET-DRAFT TRILL: Clarifications and Corrections
7. The CFI / DEI Bit
In May 2011, the IEEE promulgated [802.1Q-2011] which changes the
meaning of the bit between the priority and VLAN ID bits in the
payload of C-VLAN tags. Previously this bit was called the CFI
(Canonical Format Indicator) bit [802] and had a special meaning in
connection with IEEE 802.5 (Token Ring) frames. Now, under
[802.1Q-2011], it is a DEI (Drop Eligibility Indicator) bit, similar
to that bit in S-VLAN / B-VLAN tags where this bit has always been a
DEI bit.
The TRILL base protocol specification [RFC6325] assumed, in effect,
that the link by which end stations are connected to TRILL Switches
and the restricted virtual link provided by the TRILL Data frame are
IEEE 802.3 Ethernet links on which the CFI bit is always zero. Should
an end station be attached by some other type of link, such as a
Token Ring link, [RFC6325] implicitly assumed that such frames would
be canonicalized to 802.3 frames before being ingressed and
similarly, on egress, such frames would be converted from 802.3 to
the appropriate frame type for the link. Thus, [RFC6325] required
that the CFI bit in the Inner.VLAN always be zero.
However, for TRILL Switches with ports conforming to the change
incorporated in the IEEE 802.1Q-2011 standard, the bit in the
Inner.VLAN, now a DEI bit, MUST be set to the DEI value provided by
the EISS (Enhanced Internal Sublayer Service) interface on ingressing
a native frame. Similarly, this bit MUST be provided to the EISS
when transiting or egressing a TRILL Data frame. As with the 3-bit
priority field, the DEI bit to use in forwarding a transit frame MUST
be taken from the Inner.VLAN. The exact effect on the Outer.VLAN DEI
and priority bits and whether or not an Outer.VLAN appears at all on
the wire for output frames may depend on output port configuration.
TRILL campuses with a mixture of ports, some compliant with
[802.1Q-2011] and some compliant with pre-802.1Q-2011 standards,
especially if they have actual Token Ring links, may operate
incorrectly and may corrupt data, just as a bridged LAN with such
mixed ports and links would.
D. Eastlake, et al [Page 19]
INTERNET-DRAFT TRILL: Clarifications and Corrections
8. Graceful Restart
TRILL Switches SHOULD support the features specified in [RFC5306]
which describes a mechanism for a restarting IS-IS router to signal
to its neighbors that it is restarting, allowing them to reestablish
their adjacencies without cycling through the down state, while still
correctly initiating link state database synchronization.
D. Eastlake, et al [Page 20]
INTERNET-DRAFT TRILL: Clarifications and Corrections
9. Updates to RFC 6327
[RFC6327] provides for multiple states of the potential adjacency
between two TRILL Switches. It makes clear that only an adjacency in
the "Report" state is reported in LSPs. LSP synchronization (LSP and
SNP transmission and receipt), however, is performed if and only if
there is at least one adjacency on the link in either the "Two-Way"
or "Report" state.
To support the PORT-TRILL-VER sub-TLV specified in [RFC6326bis], the
following updates are made to [RFC6327]:
1. The first sentence of the last paragraph in [RFC6327] Section
3.1 is modified from
"All TRILL LAN Hellos issued by an RBridge on a particular
port MUST have the same source MAC address, priority,
desired Designated VLAN, and Port ID, regardless of the VLAN
in which the Hello is sent."
to
"All TRILL LAN Hellos issued by an RBridge on a particular
port MUST have the same source MAC address, priority,
desired Designated VLAN, Port ID, and PORT-TRILL-VER sub-TLV
[RFC6326bis] if included, regardless of the VLAN in which
the Hello is sent."
2. An additional bullet item is added to the end of [RFC6327]
Section 3.2 as follows:
o The 5 bytes of data from the PORT-TRILL-VER received in
the most recent TRILL Hello from the neighbor RBridge.
3. In [RFC6327] Section 3.3, near the bottom of page 12, a bullet
item as follows is added:
o The five bytes of PORT-TRILL-VER data are set from that
sub-TLV in the Hello or set to zero if that sub-TLV does
not occur in the Hello.
4. At the beginning of [RFC6327] Section 4, a bullet item is added
to the list as follows:
o The five bytes of PORT-TRILL-VER sub-TLV data used in
TRILL Hellos sent on the port.
D. Eastlake, et al [Page 21]
INTERNET-DRAFT TRILL: Clarifications and Corrections
10. Updates on Appointed Forwarders and Inhibition
An optional method of Hello reduction is specified in Section 10.1
below and a recommendation on forwarder appointments in the face of
overload is given in Section 10.2.
10.1 Optional TRILL Hello Reduction
If a network manager has sufficient confidence that they know the
configuration of bridges, ports, and the like, within a link, they
may be able to reduce the number of TRILL Hellos sent on that link;
for example, if all RBridges on the link will see all Hellos
regardless of VLAN constraints, Hellos could be sent on fewer VLANs.
However, because adjacencies are established in the Designated VLAN,
an RBridge MUST always attempt to send Hellos in the Designated VLAN.
Hello reduction makes TRILL less robust in the face of decreased VLAN
connectivity in a link such as partitioned VLANs, many VLANs disabled
on ports, or disagreement over the Designated VLAN; however, as long
as all RBridge ports on the link are configured for the same desired
Designated VLAN, can see each other's frames in that VLAN, and
utilize the mechanisms specified below to update VLAN inhibition
timers, operations will be safe. (These considerations do not arise
on links between RBridges that are configured as point-to-point
since, in that case, each RBridge sends point-to-point Hellos, other
TRILL IS-IS PDUs, and TRILL Data frames only in what it believes to
be the Designated VLAN of the link and no native frame end-station
service is provided.)
The provision for a configurable set of "Announcing VLANs", as
described in Section 4.4.3 of [RFC6325] provides a mechanism in the
TRILL base protocol for a reduction in TRILL Hellos.
To maintain loop safety in the face of occasional lost frames,
RBridge failures, link failures, new RBridges coming up on a link,
and the like, the inhibition mechanism specified in [RFC6439] is
still required. Under Section 3 of [RFC6439], a VLAN inhibition timer
can only be set by the receipt of a Hello sent or received in that
VLAN. Thus, to safely send a reduced number of TRILL Hellos on a
reduced number of VLANs requires additional mechanisms to set the
VLAN inhibition timers at an RBridge, thus extending Section 3, Item
4, of [RFC6439]. Two such mechanisms are specified below. Support for
both of these mechanisms is indicated by a capability bit in the
PORT-TRILL-VER sub-TLV (see Section 9 above and [RFC6326bis]). It may
be unsafe for an RBridge to send TRILL Hellos on fewer VLANs than the
set of VLANs recommended in [RFC6325] on a link unless all its
adjacencies on that link (excluding those in the Down state
[RFC6327]) indicate support of these mechanisms and these mechanisms
are in use.
D. Eastlake, et al [Page 22]
INTERNET-DRAFT TRILL: Clarifications and Corrections
1. An RBridge RB2 MAY include in any TRILL Hello an Appointed
Forwarders sub-TLV [RFC6326bis] appointing itself for one or more
ranges of VLANs. The Appointee Nickname field(s) in the Appointed
Forwarder sub-TLV MUST be the same as the Sender Nickname in the
Special VLANs and Flags sub-TLV in the TRILL Hello. This indicates
the sending RBridge believes it is Appointed Forwarder for those
VLANs. An RBridge receiving such a sub-TLV sets each of its VLAN
inhibition timers for every VLAN in the block or blocks listed in
the Appointed Forwarders sub-TLV to the maximum of its current
value and the Holding Time of the Hello containing the sub-TLV.
This is backwards compatible because such sub-TLVs will have no
effect on any receiving RBridge not implementing this mechanism
unless RB2 is the DRB (Designated RBridge) sending Hello on the
Designated VLAN in which case, as specified in [RFC6439], RB2 MUST
include in the Hello all forwarder appointments, if any, for
RBridges other than itself on the link.
2. An RBridge MAY use the new VLANs Appointed sub-TLV [RFC6326bis].
When RB1 receives a VLANs Appointed sub-TLV in a TRILL Hello from
RB2 on any VLAN, RB1 updates the VLAN inhibition timers for all
the VLANs that RB2 lists in that sub-TLV as VLANs for which RB2 is
Appointed Forwarder. Each such timer is updated to the maximum of
its current value and the Holding Time of the TRILL Hello
containing the VLANs Appointed sub-TLV. This sub-TLV will be an
unknown sub-TLV to RBridge not implementing it and such RBridges
will ignore it. Even if a TRILL Hello send by the DRB on the
Designated VLAN includes one or more VLANs Appointed sub-TLVs, as
long as no Appointed Forwarders sub-TLVs appear, the Hello is not
required to indicate all forwarder appointments.
Two different encodings are providing above to optimize the listing
of VLANs. Large blocks of contiguous VLANs are more efficiently
encoded with the Appointed Forwarders sub-TLV and scattered VLANs are
more efficiently encoded with the VLANs Appointed sub-TLV. These
encodings may be mixed in the same Hello. The use of these sub-TLVs
does not affect the requirement that the "AF" bit in the Special
VLANs and Flags sub-TLV MUST be set if the originating RBridge
believes it is Appointed Forwarder for the VLAN in which the Hello is
sent. If the above mechanisms are used on a link, then each RBridge
on the link MUST send Hellos in one or more VLANs with such VLANs
Appointed sub-TLV(s) and/or self-appointment Appointed Forwarders
sub-TLV(s) and the "AF" bit appropriately set such that no VLAN
inhibition timer will improperly expire unless three or more Hellos
are lost. For example, an RBridge could announce all VLANs for which
it believes it is Appointed Forwarder in a Hello sent on the
Designated VLAN three times per Holding Time.
D. Eastlake, et al [Page 23]
INTERNET-DRAFT TRILL: Clarifications and Corrections
10.2 Overload and Appointed Forwarders
An RBridge in overload (see Section 2) will, in general, do a poorer
job of ingressing and forwarding frames than an RBridge not in
overload that has full knowledge of the campus topology. For example,
an overloaded RBridge may not be able to distribute multi-destination
TRILL Data frames at all.
Therefore, the DRB SHOULD NOT appoint an RBridge in overload as an
Appointed Forwarder unless there is no alternative. Furthermore, if
an Appointed Forwarder becomes overloaded, the DRB SHOULD re-assign
VLANs from the overloaded RBridge to another RBridge on the link that
is not overloaded, if one is available. DRB election is not affected
by overload.
A counter-example would be if all campus end stations in VLAN-x were
on links attached to RB1 via ports where VLAN-x was enabled. In such
a case, RB1 SHOULD be made the VLAN-x Appointed Forwarder on all such
links even if RB1 is overloaded.
D. Eastlake, et al [Page 24]
INTERNET-DRAFT TRILL: Clarifications and Corrections
11. IANA Considerations
The following IANA actions are required:
1. The nickname 0xTBD [0xFFC1 suggested], which was reserved by
[RFC6325], is allocated for use in the TRILL Header egress
nickname field to indicate an OOMF (Overload Originated Multi-
destination Frame).
2. Bit 1 from the seven previously reserved (RESV) bits in the per
neighbor "Neighbor RECORD" in the TRILL Neighbor TLV [RFC6326bis]
is allocated to indicate that the RBridge sending the TRILL Hello
volunteers to provide the OOMF forwarding service described in
Section 2.4.2 to such frames originated by the TRILL Switch whose
SNPA (MAC address) appears in that Neighbor RECORD.
3. Bit 0 is allocated from the Capability bits in the PORT-TRILL-VER
sub-TLV [RFC6326bis] to indicate support of the VLANs Appointed
sub-TLV [RFC6326bis] and the VLAN inhibition setting mechanisms
specified in Section 10.1.
D. Eastlake, et al [Page 25]
INTERNET-DRAFT TRILL: Clarifications and Corrections
12. Security Considerations
This memo improves the documentation of the TRILL protocol, corrects
five errata in [RFC6325], and updates [RFC6325], [RFC6327], and
[RFC6439]. It does not change the security considerations of these
RFCs.
Acknowledgements
The contributions of the following persons are gratefully
acknowledged:
Somnath Chatterjee, Weiguo Hao, Rakesh Kumar, Yizhou Li, Radia
Perlman, Mike Shand, Meral Shirazipour, and Varun Varshah.
This document was produced with raw nroff. All macros used were
defined in the source file.
Change History
RFC Editor Note: Please delete this Change History section before
publication.
Changes from -01 to -02:
Add Section 3.4. Minor editorial changes and update of the version
and dates.
Changes from -02 to -03:
Emphasize in Section 3.4 that the change is not backwards
compatible and that all the RBridges in a campus must calculate
the distribution trees the same way or RPFC will discard too much.
Provide additional detail at the end of Item 5, Section 4, as to
what Channel messages can be received by an RBridge without a
nickname. Add Section 1.2 on incompatible changes, renumbering the
former Section 1.2 as 1.3.
Changes from -03 to -04:
Update author information. Numerous typos fixes and editorial
changes based on the GENART review and various Area Director
reviews. The update from -03 to -04 is not intended to make any
technical change.
Changes from -04 to -05:
Change text to make it clear that frames are not least cost routed
through overloaded RBridges. Minor editorial changes, some to
resolve second-pass GENART comments.
D. Eastlake, et al [Page 26]
INTERNET-DRAFT TRILL: Clarifications and Corrections
Changes from -05 to -06:
Improve wording of Section 4, bullet 4, and Section 5, paragraph
2, as per suggestions by Mike Shand. Fix typos in Change History.
D. Eastlake, et al [Page 27]
INTERNET-DRAFT TRILL: Clarifications and Corrections
Normative References
[802.1Q-2011] - IEEE 802.1, "IEEE Standard for Local and metropolitan
area networks - Virtual Bridged Local Area Networks", IEEE Std
802.1Q-2011, May 2011.
[Err3002] - RFC Errata, Errata ID 3002, RFC 6325, http://www.rfc-
editor.org.
[Err3003] - RFC Errata, Errata ID 3003, RFC 6325, http://www.rfc-
editor.org.
[Err3004] - RFC Errata, Errata ID 3004, RFC 6325, http://www.rfc-
editor.org.
[Err3052] - RFC Errata, Errata ID 3052, RFC 6325, http://www.rfc-
editor.org.
[Err3053] - RFC Errata, Errata ID 3053, RFC 6325, http://www.rfc-
editor.org.
[IS-IS] - ISO/IEC 10589:2002, Second Edition, "Intermediate System to
Intermediate System Intra-Domain Routeing Exchange Protocol for
use in Conjunction with the Protocol for Providing the
Connectionless-mode Network Service (ISO 8473)", 2002.
[RFC1195] - Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5305] - Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC5306] - Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS",
RFC 5306, October 2008.
[RFC6325] - Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC6327] - Eastlake 3rd, D., Perlman, R., Ghanwani, A., Dutt, D.,
and V. Manral, "Routing Bridges (RBridges): Adjacency", RFC
6327, July 2011.
[RFC6439] - Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
Hu, "Routing Bridges (RBridges): Appointed Forwarders", RFC
6439, November 2011.
D. Eastlake, et al [Page 28]
INTERNET-DRAFT TRILL: Clarifications and Corrections
[RFC6326bis] - Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and
A. Ghanwani, draft-eastlake-isis-rfc6326bis, work in progress.
Informative References
[802] - IEEE 802, "IEEE Standard for Local and metropolitan area
networks: Overview and Architecture", IEEE Std 802.1-2001, 8
March 2002.
[Channel] - Eastlake, E., Manral, V., Li, Y., Ward, D., draft-ietf-
trill-rbridge-channel, work in progress.
[RFC5342] - Eastlake 3rd, D., "IANA Considerations and IETF Protocol
Usage for IEEE 802 Parameters", BCP 141, RFC 5342, September
2008.
D. Eastlake, et al [Page 29]
INTERNET-DRAFT TRILL: Clarifications and Corrections
Authors' Addresses
Donald Eastlake
Huawei R&D USA
155 Beaver Street
Milford, MA 01757 USA
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com
Mingui Zhang
Huawei Technologies Co., Ltd
Huawei Building, No.156 Beiqing Rd.
Z-park, Shi-Chuang-Ke-Ji-Shi-Fan-Yuan, Hai-Dian District,
Beijing 100095 P.R. China
Email: zhangmingui@huawei.com
Anoop Ghanwani
Dell
350 Holger Way
San Jose, CA 95134 USA
Phone: +1-408-571-3500
Email: anoop@alumni.duke.edu
Vishwas Manral
HP Networking
19111 Pruneridge Avenue
Cupertino, CA 95014 USA
Tel: +1-408-477-0000
Email: vishwas.manral@hp.com
Ayan Banerjee
Cumulus Networks
1089 West Evelyn Avenue
Sunnyvale, CA 94086 USA
Email: ayabaner@gmail.com
D. Eastlake, et al [Page 30]
INTERNET-DRAFT TRILL: Clarifications and Corrections
Copyright and IPR Provisions
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This document is subject to BCP 78 and the IETF Trust's Legal
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language to the contrary, or terms, conditions or rights that differ
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RFC 5378, shall have any effect and shall be null and void, whether
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Contribution.
D. Eastlake, et al [Page 31]
